1. Field of the Invention
This invention relates to optoelectronic detector arrays and more specifically to Focal Plane Arrays (FPAs).
2. Description of the Related Art
A photodetector registers photon flux striking it as a function of time. An array of a large number of photodetectors can simultaneously register the photon fluxes from many spatial points (called pixels) to form an electronic version of an optical image. In such arrays, some detectors will often be inoperable due to randomly located material defects such as structural dislocations and processing defects that cause electrical shorts or unacceptably high tunneling currents, resulting in poor image uniformity.
Smaller photodetectors reduce the impact of defects which are due to structural dislocations, since the detectors cover a smaller area of the material layer and have a reduced chance of being located in a defective region. However, smaller detectors also have a decreased optical collection efficiency. Focal Plane Arrays (FPAs) use microlenses which concentrate incident radiation into a detector region, thereby permitting smaller area detectors without significant loss of optical collection efficiency. (see Motamedi, et. al., FPA""s and Thin Film Binary Optic Microlens Integration, Miniaturized Systems with Micro-Optics and Micromechanics, Volume 2687, January 1996, page 70). Using this technique, optical collection efficiency or operability in the 90-98% range has been achieved for strategic Very long wavelength Infrared (VLWIR) FPAs operating at 40xc2x0 K. Still, for many applications including hyperspectral imaging using VLWIR FPAs, clusters of bad pixels are deemed unacceptable because either spatial or spectral information in a critical band may be lost. Due to the high random nature of the defects, achieving low or zero cluster outages requires an extremely high operability, generally 99% or greater, which is presently not achieved even with the use of microlens arrays.
The present invention is an FPA array, and method for fabricating an FPA array, having a plurality of detectors per pixel, wherein radiation is directed to the best detector of each pixel.
According to a preferred method, the FPA has a microlens array which is custom designed to focus and direct radiation to the most operative detector in each pixel. Each detector is connected to a separate and selectable input of a multiplexer (MUX). Since FPAs are generally tested before the fabrication of the microlens array, the operability of each detector can be evaluated, and a file generated specifying detector operability for each pixel. According to one embodiment, this file is used to generate a custom gray-scale microlens mask for fabricating the array using photolithography, in which the shape of the lens for each pixel is chosen to direct the lens focus spot to the best detector in each pixel. The better detector is similarly selected for readout by the MUX while the other detectors are not selected.
This reduces the impact of both material and fabrication defects, since detectors which are defective for any reason can be screened out. Thus, for an array having two detectors per pixel, if the probability of any one detector being defective is 2-10%, the probability of both detectors per pixel being defective is approximately 0.04-1%.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings, in which: